Tetrapod Zoology

Tetrapod Zoology

Amphibians, reptiles, birds and mammals - living and extinct

Phenacodontidae, I feel like I know you


One day in the not-too-distant future there will be a Tet Zoo Guide to Paleogene Mammals. I'm not kidding - it really will happen one day... hey, I've only been promising it since 2007 or so. Until then, here's a very short excerpt from that project (YES, it does exist, as does tons and tons and tons of stuff that I just never have time to finish).

Phenacodus primaevus, photographed at the AMNH by Ghedoghedo. Image in public domain.

An enormous number of Paleogene eutherians have traditionally been grouped together in Condylarthra, a group that represents a classic ‘taxonomic wastebasket’ (that is, a storage area for taxa that can’t confidently be placed elsewhere). At the core of the condylarth concept are the phenacodontids, periptychids and hyopsodontids but numerous other Paleogene groups later became added, including arctocyonids and mesonychians. Affinities with such groups as pantodonts, taeniodonts, tillodonts, cetaceans and aardvarks have also been suggested. Anatomical characters that might link all of these animals to the exclusion of other eutherians are non-existent and hence there has been widespread recognition of the fact that Condylarthra is a grade, not a clade.

Cranial and mandibular remains of the phenacodontid Ectocion osbornianus (one of several Ectocion species), from Thewissen (1990).

However, occasional evidence has been presented that might link certain condylarth groups together. Furthermore, large-scale phylogenetic analyses have also led to proposals that some condylarth lineages might be nested within eutherian clades originally recognised following molecular studies, most notably Afrotheria (Tabuce et al. 2007). Tabuce et al. (2001) drew attention to a possible close relationship between a Microhyus + Macroscelidea clade and Proboscidea, Hyracoidea, Perissodactyla and Phenacodontidae. A clade containing all of these taxa corresponds to the group Taxeopoda recognised by Archibald (1998): it is mostly congruent with molecular data apart from the inclusion of perissodactyls. For reviews of the condylarth concept see Prothero et al. (1988) and Archibald (1998).

Phenacodontids are one of those ‘classic’ Paleogene eutherian groups that get featured in the majority of books on fossil mammals; this is due to the fact that Phenacodus from the Paleocene and Eocene of North America and Europe is represented by near-complete remains, and that it has often been imagined as possibly close to the ancestry of perissodactyls. Indeed, phenacodontids share several ankle, humeral and braincase characters with tethytheres and perissodactyls (Prothero et al. 1988).

Charles Knight's 1898 reconstruction of Phenacodus. The flora perhaps looks a bit too modern. Image in public domain.

Possible phenacodontids have been reported from the Paleocene of South America and Eocene of Asia but these records were considered doubtful by Thewissen (1990). Phenacodontids have bunodont or bunolophodont cheek teeth, spatulate lower incisors and large, pointed canines. They were reasonably long-bodied, long-tailed mammals with pentadactyl hoofed hands and feet; their proportions and body size are indicative of semi-cursoriality. Sexual dimorphism in canine size is present in some members of the group and the snout morphology of one species (Phenacodus intermedius) suggests the presence of a short proboscis (Thewissen 1990).

For previous Tet Zoo articles on Paleogene mammals, see...

Refs - -

Archibald, J. D. 1998. Archaic ungulates (“Condylarthra”). In Janis, C. M., Scott, K. M. & Jacobs, L. L. (eds) Evolution of Tertiary Mammals of North America. Volume 1: Terrestrial Carnivores, Ungulates, and Ungulatelike Mammals. Cambridge University Press, pp. 292-331.

Prothero, D. R., Manning, E. M. & Fischer, M. 1988. The phylogeny of the ungulates. In Benton, M. J. (ed) The Phylogeny and Classification of the Tetrapods, Volume 2: Mammals. Clarendon Press (Oxford), pp. 201-234.

Tabuce, R., Coiffait, B., Coiffait, P.-E., Mahboubi, M. & Jaeger, J.-J. 2001. A new genus of Macroscelidea (Mammalia) from the Eocene of Algeria: a possible origin for elephant-shrews. Journal of Vertebrate Paleontology 21, 535-546.

- ., Marivaux, L., Adaci, M., Bensalah, M., Hartenberger, J.-L., Mahboubi, M., Mebrouk, F., Tafforeau, P. & Jaeger, J.-J. 2007. Early Tertiary mammals from North Africa reinforce the molecular Afrotheria clade. Proceedings of the Royal Society B 274, 1159-1166.

Thewissen, J. G. M. 1990. Evolution of Paleocene and Eocene Phenacodontidae (Mammalia, Condylarthra). University of Michigan Papers on Paleontology 29, 1-107.

The views expressed are those of the author and are not necessarily those of Scientific American.

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